The invention relates to a method for coating an optoelectronic chip-on-board module, which comprises a flat substrate populated with one or more optoelectronic components, having a transparent, UV-resistant, and temperature-resistant coating made of one or more silicones. The invention also relates to a corresponding optoelectronic chip-on-board module and to a system having several optoelectronic chip-on-board modules.
Optoelectronic chip-on-board modules of this general class are used, for example, as illuminating elements, as high-power UV LED lamps, as photovoltaic modules, as sensors, or the like. The optoelectronic components used here are, in the scope of the invention, but not exclusively, LEDs or photodiodes in the form of chips or other components arranged in the chip-on-board module on a flat substrate, that is, a metal, ceramic, or silicon substrate, a metal core or FR4 circuit board, a glass substrate, a plastic substrate, or the like. These chip-on-board modules must be protected from mechanical damage and corrosion. For this purpose, the most compact and light-weight solutions possible are desired.
A protection in the form of housings on chip-on-board modules is often expensive and technologically complicated. One practical alternative for the protection of chip-on-board modules is a flat encapsulation of the components with a plastic-based encapsulation or casting material. Together with other functional components, such as circuit boards and contacting elements, the optoelectronic components in chip-on-board modules are protected, together with a flat substrate, from mechanical damage and corrosion by coatings.
Typically, epoxy resins are used for this purpose. These are deposited as an encapsulation material, initially as a fluid and then cured by heat and/or radiation. Since the encapsulation material is initially a fluid, the encapsulation material must be prevented from flowing away. This is typically realized by a mold or a fixed frame.
The so-called “dam and fill” method forms one alternative here, wherein initially a plastic dam is applied on the substrate of the chip-on-board module, the dam enclosing an area of the substrate in which a fluid filling compound made of epoxy resin is then filled. This compound is cured. The dam and filling compound together form the coating of the module. For generating the dam, in this method, a viscous polymer is deposited with a dispensing device or drawn out and then cured, so that encapsulation material can be cast on the surface enclosed by the dam, without this material flowing away.
The plastic dam generated in this manner, however, is not transparent. Therefore, the luminous radiation intensity or light sensitivity of optoelectronic chip-on-board modules coated in this manner, that is, chip-on-board modules populated with optoelectronic components, such as LEDs or photodiodes, is adversely affected toward the edge.
One class of additional materials that can be used as a dam in a “dam and fill” method is the class of thixotropic epoxy resins. These are used for this purpose, for example, in the manufacture of chip cards. Thixotropic epoxy resins are treated so that their viscosity depends on the mechanical force application and its duration. Therefore, the application of force on the dam liquefies the thixotropic epoxy resins, and the relaxation of the material after emerging from a nozzle subsequently solidifies it. Thus, they are well suited for generating a stable dam in a “dam and fill” method. Epoxy resins, however, are not UV-resistant and therefore are unstable in a high-power UV LED module or also under intense solar irradiation with UV components, like those in photovoltaic cells. They age quickly under UV loading and are destroyed.
Up to now no method for realizing a flat coating for chip-on-board modules has been known in which materials, which are both UV-resistant and also temperature-resistant and are likewise transparent for electromagnetic radiation from the ultraviolet to the infrared spectral range, are used both in the surface areas and also the edge areas of the encapsulation material.
Other solutions, for example the adhesion of a glass frame or a glass dome, which are transparent, UV-resistant, and temperature-resistant, require a very complicated installation of the frame and a compactness of the frame that is hard to produce. Such a solution is also associated with a greater weight than a “dam and fill” solution. For rigid glass materials, a usually required adaptation of the thermal expansion coefficients of the composite materials also represents another hurdle, especially when the subsequent products are exposed to thermal cycles.
For a combination solution made of a glass frame and an encapsulation with a suitable non-epoxy resin-based material, such as a temperature-resistant and UV-resistant silicone, very small gaps between the frame and substrate can have the effect that the silicone, which has strong creeping properties, can run out during the casting. Space for the frame also must be provided on the substrate. This adversely affects the best possible utilization of the substrate surface and/or a desired stackability.
For the use of chip-on-board technology for the production of high-power UV LED modules that emit over a planar area or of photodiode arrays, a flat encapsulation that avoids the mentioned disadvantages is advantageous. For reasons of optical efficiency and best possible stackability of modules, the encapsulation should have transparent faces and also edge areas. High temperature resistance and UV resistance are likewise relevant both for the production of corresponding optoelectronic components and also for long-term, stable functioning.